Data processing systems often have the capability of performing a potentially bewildering variety of preselected processing operations upon an equally large variety of types of data inputted to the system by a human operator. For example, in a data processing system which is designed to produce a building construction cost estimate from information inputted to the system by a human operator working from architects drawings or blueprints, the system must be capable of (1) accepting quantitative data concerning a vast array of purchased hardware such as plumbing and electrical fittings, (2) accessing its relevant cost data concerning each item of equipment inputted, and (3) producing an accurate total cost for the entire construction project.
Similarly the system operator (or estimator) in dealing with the great variety of different types of data which go into the cost estimate requires a simple means to inform him in human language of the function of each control on the computer input/output station no matter which mode of system operation is in use.